Heretofore, it has been known to employ a number of pretreatment or sensitization baths in effecting the electroless deposition of metals on various surfaces. Typically, such prior art sensitization baths used commercially have been expensive because they depend upon a noble metal, e.g., Pd, Pt, Ag, Au, etc., as the catalytic component for electroless deposition. However, more recently, methods have been reported in which electroless metal deposits can be applied to a broad variety of insulating substrates without the need to use expensive noble metals. These recent methods employ reducible salt compositions of non-noble metals. Examples of such methods can be found with reference to U.S. Pat. Nos. 3,772,056; 3,772,078; 3,907,621; 3,925,578; 3,930,963; 3,993,802; and 3,994,727. A problem with the methods disclosed in these patents and not recognized or addressed thereby or therein is the storage time generally required after sensitization of the substrate. Typical storage times of these prior art methods of a substrate having a sensitized surface thereon, prior to imaging of that surface, is in the order of hours. During production of printed wiring boards, however, one often encounters the need for a significantly longer storage time between the step of sensitizing the substrate surface and imaging that surface to obtain a pattern thereon. This requirement generally comes about due to the fact that the imaging processing step is generally much more time consuming than the step of forming the sensitized surface on the substrate, which latter step can be performed in large batches. There is therefore often a backup and holding or inventory period between the formation of the sensitized substrate and the imaging and further processing thereof. Consequently, it is a requirement during actual commercial use of such processes to have a sensitized substrate which can be stored for several days without degradation of the sensitization layer. It is desirable to maintain storage capabilities of at least 7 days.
Recently issued U.S. Pat. No. 4,098,922 does address itself, in part, to the aging or storage problem. The solution to the problem proposed therein is the use of lactose in the formulation without drying the sensitizing solution. It has been found, however, that imaging requires a certain amount of moisture and if this moisture is not present, no or little imaging occurs. Since lactose does not retain any significant quantity of water, lactose alone is only suitable when imaging is performed in high humidity conditions, e.g., greater than 75% humidity. Where the humidity is 75% or less, then another, water retaining polyol may be used in combination with the lactose. However, in this instance, shelf lives of only 1-2 days can be expected in humidities of about 50-70%.
The prior art suggests sensitization solutions including many possible alternative compounds and compositions. More particularly, the aforementioned patents suggest dozens of compounds, which are recognized only as mere alternatives, e.g., compounds selected from ferric salts, bichromate salts and anthraquinone and its derivatives. Included as a mere alternative are salts of anthraquinone 2,7 disulfonic acids. The prior art, however, does not recite any specific example employing this compound. We have surprisingly found that this one specific derivative of anthraquinone disulfonic acid, when used in combination with the other ingredients of the sensitization solutions previously disclosed, results in a sensitized layer having much longer storage times without degradation than that resulting from the use of prior art solutions and methods in almost any humidity conditions.